Nitrogen is one of three essential chemicals needed for plant growth. Anhydrous ammonia, NH.sub.3, contains a high percentage, 82% by weight, of nitrogen that is in available form for utilization by plants, as corn and the like. It is current agricultural practice to inject anhydrous ammonia into the soil with an implement having a plurality of knived injectors. Anhydrous ammonia being a liquid under pressure has a relatively low boiling point, making the NH.sub.3 difficult to measure and control. In practice, the rate of application of NH.sub.3 can vary up to 15% between the time the injection is started with a full anhydrous ammonia tank and the time the injection is completed. The variation in application rate is caused by temperature changes inside the tank as it is emptied. A second factor affecting the application rate of NH.sub.3 is the ground speed of the implement. Varying soil conditions, field terrain, and types of tillage equipment all affect the towing vehicle's ability to maintain a constant ground speed. The combined temperature variations and ground speed variation can cause 20% to 40% error in injection application of NH.sub.3. The lack of effective control and measurement of the application rate of NH.sub.3 results in either an over-application or under-application of the NH.sub.3 into the soil. This is an inefficient utilization of the NH.sub.3 and can cause substantial reductions in crop yields.